Color separation in planar waveguides using wavelength filters

ABSTRACT

An eyepiece for projecting an image to an eye of a viewer includes a first planar waveguide positioned in a first lateral plane, a second planar waveguide positioned in a second lateral plane adjacent the first lateral plane, and a third planar waveguide positioned in a third lateral plane adjacent the second lateral plane. The first waveguide includes a first diffractive optical element (DOE) coupled thereto and disposed at a lateral position. The second waveguide includes a second DOE coupled thereto and disposed at the lateral position. The third waveguide includes a third DOE coupled thereto and disposed at the lateral position. The eyepiece further includes a first optical filter disposed between the first waveguide and the second waveguide at the lateral position, and a second optical filter positioned between the second waveguide and the third waveguide at the lateral position.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/445,115 filed on Jun. 18, 2019, now U.S. Pat. No. 10,551,568 issuedon Feb. 4, 2020, entitled “EYEPIECE PROVIDING COLOR SEPARATION IN PLANARWAVEGUIDES USING DICHROIC FILTERS,” which is a divisional of U.S. patentapplication Ser. No. 15/849,527 filed on Dec. 20, 2017, now U.S. Pat.No. 10,371,896 issued on Aug. 6, 2019, entitled “COLOR SEPARATION INPLANAR WAVEGUIDES USING DICHROIC FILTERS,” which is a non-provisional ofand claims priority to U.S. Provisional Patent Application No.62/438,315 filed on Dec. 22, 2016, entitled “COLOR SEPARATION INWAVEGUIDES USING DICHROIC FILTERS,” the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Modern computing and display technologies have facilitated thedevelopment of systems for so called “virtual reality” or “augmentedreality” experiences, wherein digitally reproduced images or portionsthereof are presented to a viewer in a manner wherein they seem to be,or may be perceived as, real. A virtual reality, or “VR,” scenariotypically involves presentation of digital or virtual image informationwithout transparency to other actual real-world visual input; anaugmented reality, or “AR,” scenario typically involves presentation ofdigital or virtual image information as an augmentation to visualizationof the actual world around the viewer.

Despite the progress made in these display technologies, there is a needin the art for improved methods and systems related to augmented realitysystems.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, an eyepiece forprojecting an image to an eye of a viewer includes a first planarwaveguide positioned in a first lateral plane, a second planar waveguidepositioned in a second lateral plane adjacent the first lateral plane,and a third planar waveguide positioned in a third lateral planeadjacent the second lateral plane. The first waveguide includes a firstdiffractive optical element (DOE) coupled thereto and disposed at alateral position. The first DOE is configured to diffract image light ina first wavelength range centered at a first wavelength. The secondwaveguide includes a second DOE coupled thereto and disposed at thelateral position. The second DOE is configured to diffract image lightin a second wavelength range centered at a second wavelength longer thanthe first wavelength. The third waveguide includes a third DOE coupledthereto and disposed at the lateral position. The third DOE configuredto diffract image light in a third wavelength range centered at a thirdwavelength longer than the second wavelength. The eyepiece furtherincludes a first optical filter disposed between the first waveguide andthe second waveguide at the lateral position, and a second opticalfilter positioned between the second waveguide and the third waveguideat the lateral position. The first optical filter is configured to havea first transmittance value at the first wavelength range, a secondtransmittance value at the second wavelength range and the thirdwavelength range that is greater than the first transmittance value, anda first reflectance value at the first wavelength range that is greaterthan about 90%. The second optical filter is configured to have a thirdtransmittance value at the first wavelength range and the secondwavelength range, a fourth transmittance value at the third wavelengthrange that is greater than the third transmittance value, and a secondreflectance value at the second wavelength range that is greater thanabout 90%. In some examples, each of the first transmittance value andthe third transmittance value may be less than about 10%; and each ofthe second transmittance value and the fourth transmittance value may begreater than about 90%. In some other examples, each of the firsttransmittance value and the third transmittance value may be less thanabout 20%; and each of the second transmittance value and the fourthtransmittance value may be greater than about 80%. In some examples, thefirst optical filter may be configured to have the first transmittancevalue and the second transmittance value for angles of incidence rangingfrom about zero degree to about 45 degrees; and the second opticalfilter may be configured to have the third transmittance value and thefourth transmittance value for angles of incidence ranging from aboutzero degree to about 45 degrees. In some other examples, the firstoptical filter may be configured to have the first transmittance valueand the second transmittance value for angles of incidence ranging fromabout zero degree to about 25 degrees; and the second optical filter maybe configured to have the third transmittance value and the fourthtransmittance value for angles of incidence ranging from about zerodegree to about 25 degrees.

According to another embodiment of the present invention, an eyepiecefor projecting an image to an eye of a viewer includes a first planarwaveguide positioned in a first lateral plane. The first waveguide has afirst lateral region and a second lateral region. The first lateralregion is disposed at a lateral position and configured to receive imagelight incident on a first lateral surface thereof. The image lightincludes image light in a first wavelength range centered at a firstwavelength, image light in a second wavelength range centered at asecond wavelength longer than the first wavelength, and image light in athird wavelength range centered at a third wavelength longer than thesecond wavelength. The eyepiece further includes a first diffractiveoptical element (DOE) optically coupled to the first lateral region ofthe first waveguide and configured to diffract image light in the firstwavelength range into the first waveguide to be guided toward the secondlateral region of the first waveguide. A first portion of the imagelight is transmitted through the first waveguide. The eyepiece furtherincludes a first optical filter positioned in a second lateral planeadjacent the first lateral plane at the lateral position and configuredto receive the first portion of the image light. The first opticalfilter is further configured to have a first transmittance value for thefirst wavelength range and a second transmittance value for the secondwavelength range and the third wavelength range that is greater than thefirst transmittance value. The eyepiece further includes a second planarwaveguide positioned in a third lateral plane adjacent the secondlateral plane. The second waveguide has a first lateral region and asecond lateral region. The first region is disposed at the lateralposition and configured to receive image light transmitted through thefirst optical filter and incident at a first lateral surface thereof.The eyepiece further includes a second DOE optically coupled to thefirst lateral region of the second waveguide and configured to diffractimage light in the second wavelength range into the second waveguide tobe guided toward the second lateral region of the second waveguide. Asecond portion of the image light is transmitted through the secondwaveguide. The eyepiece further includes a second optical filterpositioned in a fourth lateral plane adjacent the third lateral plane atthe lateral position and configured to receive the second portion of theimage light. The second optical filter is configured to have a thirdtransmittance value for the first wavelength range and the secondwavelength range and a fourth transmittance value for the thirdwavelength range that is greater than the third transmittance value. Theeyepiece further includes a third planar waveguide positioned at a fifthlateral plane adjacent the fourth lateral plane. The third waveguide hasa first lateral region and a second lateral region. The first lateralregion is disposed at the lateral position and configured to receiveimage light transmitted through the second optical filter and incidentat a first lateral surface thereof. The eyepiece further includes athird DOE optically coupled to the first lateral region of the thirdwaveguide and configured to diffract image light in the third wavelengthrange into the third waveguide to be guided toward the second lateralregion of the third waveguide.

According to yet another embodiment of the present invention, aneyepiece for projecting image light to an eye of a viewer includes afirst planar waveguide. The first waveguide includes a first diffractiveoptical element (DOE) optically coupled thereto. The first DOE ispositioned along an optical path of the image light and configured tocouple a portion of the image light in a first wavelength range centeredat a first wavelength into the first planar waveguide to be propagatedin the first planar waveguide. The eyepiece further includes a firstoptical filter positioned along the optical path downstream from thefirst DOE. The first optical filter is configured to attenuate the imagelight in the first wavelength range incident thereon. The eyepiecefurther includes a second planar waveguide. The second waveguideincludes a second DOE optically coupled thereto. The second DOE ispositioned along the optical path downstream from the first opticalfilter and configured to couple a portion of the image light in a secondwavelength range centered at a second wavelength different from thefirst wavelength into the second planar waveguide to be propagated inthe second planar waveguide. The eyepiece further includes a secondoptical filter coupled to the first planar waveguide. The second opticalfilter is configured to absorb image light in the second wavelengthrange propagating in the first planar waveguide.

According to a further embodiment of the present invention, an eyepiecefor projecting an image to an eye of a viewer includes a first planarwaveguide positioned in a first lateral plane, a second planar waveguidepositioned in a second lateral plane adjacent the first lateral plane,and a third planar waveguide positioned in a third lateral planeadjacent the second lateral plane. The first waveguide includes a firstdiffractive optical element (DOE) coupled thereto and disposed at afirst lateral position. The second waveguide includes a second DOEcoupled thereto and disposed at a second lateral position. The thirdwaveguide includes a third DOE coupled thereto and disposed at thesecond lateral position. The eyepiece further includes an optical filterpositioned between the second waveguide and the third waveguide at thesecond lateral position.

According to some other embodiments of the present invention, aneyepiece for projecting an image to an eye of a viewer includes a firstplanar waveguide positioned in a first lateral plane. The firstwaveguide includes a first incoupling element optically coupled thereto.The first incoupling element is configured to diffract image light in afirst wavelength range centered at a first wavelength. The eyepiecefurther includes a second planar waveguide positioned in a secondlateral plane adjacent the first lateral plane. The second waveguideincludes a second incoupling element optically coupled thereto. Thesecond incoupling element is configured to diffract image light in asecond wavelength range centered at a second wavelength different fromthe first wavelength. The eyepiece further includes a first opticalelement positioned between the first waveguide and the second waveguidein lateral alignment with the first incoupling element. The firstoptical element is configured to reflect image light in the firstwavelength range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the light paths in a part of a viewingoptics assembly (VOA) that may be used to present a digital or virtualimage to a viewer, according to an embodiment of the present invention.

FIG. 2 illustrates schematically one method of color separation in aneyepiece for viewing a virtual image.

FIG. 3 illustrates schematically another method of color separation inan eyepiece for viewing a virtual image according to an embodiment ofthe present invention.

FIG. 4 illustrates schematically a plan view of an eyepiece according toan embodiment of the present invention.

FIG. 5 illustrates schematically a partial cross-sectional view of aneyepiece according to an embodiment of the present invention.

FIG. 6A-6D illustrate some example images formed by an eyepiece withoutfilters according to an embodiment of the present invention.

FIG. 6E-6H illustrate some example images formed by an eyepiece withdichroic filters according to an embodiment of the present invention.

FIG. 7 illustrates schematically a transmittance/reflectance curve of anoptical filter according to an embodiment of the present invention.

FIG. 8 illustrates schematically a transmittance/reflectance curve of anoptical filter according to another embodiment of the present invention.

FIG. 9A illustrates schematically a partial cross-sectional view of awaveguide including a short-pass filter coupled thereto according to anembodiment of the present invention.

FIG. 9B illustrates schematically a cross-sectional view of a short-passfilter according to an embodiment of the present invention.

FIG. 10 illustrates schematically a partial cross-sectional view of awaveguide including a short-pass filter coupled thereto according to anembodiment of the present invention.

FIGS. 11A-11D illustrate the wavelength cross-coupling effect of thewaveguides in an eyepiece.

FIGS. 12A-12C illustrate schematically partial cross-sectional views ofwaveguides including a short-pass filter coupled thereto according toembodiments of the present invention.

FIGS. 13A-13C illustrate partial cross-sectional views of eyepiecesaccording to various embodiments of the present invention.

FIG. 14 illustrates schematically a transmittance/reflectance curve ofan optical filter according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present disclosure relates generally to eyepieces that may be usedfor virtual reality and augmented reality visualization systems. Moreparticularly, the present invention relates to an eyepiece that includesone or more long-pass dichroic filters for color separation betweendifferent waveguides. The eyepiece may also include one or moreshort-pass dichroic filters for further reducing wavelengthcross-coupling. Such an eyepiece may afford a more compact form factorand enhanced brightness and contrast of the light fields, as well asreduced wavelength cross-coupling, as compared to conventionaleyepieces.

FIG. 1 illustrates schematically the light paths in a part of a viewingoptics assembly (VOA) that may be used to present a digital or virtualimage to a viewer, according to an embodiment of the present invention.The VOA includes a projector 101 and an eyepiece 100 that may be wornaround a viewer's eye. In some embodiments, the projector 101 mayinclude a group of red LEDs, a group of green LEDs, and a group of blueLEDs. For example, the projector 101 may include two red LEDs, two greenLEDs, and two blue LEDs according to an embodiment. The eyepiece 100 mayinclude one or more eyepiece layers. In one embodiment, the eyepiece 100includes three eyepiece layers, one eyepiece layer for each of the threeprimary colors, red, green, and blue. In another embodiment, theeyepiece 100 may include six eyepiece layers, i.e., one set of eyepiecelayers for each of the three primary colors configured for forming avirtual image at one depth plane, and another set of eyepiece layers foreach of the three primary colors configured for forming a virtual imageat another depth plane. In other embodiments, the eyepiece 100 mayinclude three or more eyepiece layers for each of the three primarycolors for three or more different depth planes. Each eyepiece layerincludes a planar waveguide and may include an incoupling grating 107,an orthogonal pupil expander (OPE) region 108, and an exit pupilexpander (EPE) region 109.

Still referring to FIG. 1, the projector 101 projects image light ontothe incoupling grating 107 in an eyepiece layer 100. The incouplinggrating 107 couples the image light from the projector 101 into theplanar waveguide propagating in a direction toward the OPE region 108.The waveguide propagates the image light in the horizontal direction bytotal internal reflection (TIR). The OPE region 108 of the eyepiecelayer 100 also includes a diffractive element that couples and redirectsa portion of the image light propagating in the waveguide toward the EPEregion 109. The EPE region 109 includes an diffractive element thatcouples and directs a portion of the image light propagating in thewaveguide in a direction approximately perpendicular to the plane of theeyepiece layer 100 toward a viewer's eye 102. In this fashion, an imageprojected by projector 101 may be viewed by the viewer's eye 102. Thepart of the VOA illustrated in FIG. 1 may constitute a “monocle” for oneeye of the viewer. The entire VOA may include two such monocles, one foreach eye of the viewer.

As described above, image light generated by the projector may includelight in the three primary colors, namely blue (B), green (G), and red(R). Such image light will need to be separated into the constituentcolors, so that image light in each constituent color may be coupled toa respective waveguide in the eyepiece. FIG. 2 illustrates schematicallyone method of color separation using a “split pupil” approach. In thisexample, an eyepiece 230 includes a blue waveguide 240, a greenwaveguide 250, and a red waveguide 260. Each waveguide 240, 250, or 260may include an incoupling grating (ICG) 242, 252, or 262, an orthogonalpupil expander (OPE) region 244, 254, or 264, and an exit pupil expander(EPE) region 246, 256, or 266. The ICG, the OPE, and the EPE in eachwaveguide are designed for a particular wavelength range. For example,the ICG 242 in the blue waveguide 240 may include a diffractive opticalelement (DOE) configured to diffract primarily blue light into the bluewaveguide 240 to be guided toward the OPE region 244. The OPE region 244of the blue waveguide 240 may include a DOE configured to diffractprimarily blue light toward the EPE region 246. The EPE region 246 ofthe blue waveguide 240 may include a DOE configured to diffractprimarily blue light toward the viewer's eye 270.

In the example illustrated in FIG. 2, a color separator 220 may separatethe image light in blue, green, and red colors generated by theprojector subsystem 210 into three spatially separate light paths: theblue light path 248, the green light path 258, and the red light path268. The ICGs 242, 252, and 262 in the blue, green, and red waveguides240, 250, and 260 may be laterally offset from each other, such that theICG 242 for the blue waveguide 240 may be aligned with the blue lightpath 248, the ICG 252 for the green waveguide 250 may be aligned withthe green light path 258, and the ICG 262 for the red waveguide 260 maybe aligned with the red light path 268. The eyepiece 230 illustrated inFIG. 2 may have a relatively large form factor as the ICGs 242, 252, and262 in the three waveguides 240, 250, and 260 need to be laterallydisplaced with respect to each other.

FIG. 3 illustrates schematically another method of color separationusing an “in-line” approach according to an embodiment of the presentinvention. In this example, the eyepiece 330 may also include a bluewaveguide 340, a green waveguide 350, and a red waveguide 360. Eachwaveguide 340, 350, or 360 may include an ICG 342, 352, or 362, an OPEregion 344, 354, or 364, and a EPE region 346, 356, or 366. Here, imagelight in blue, green, and red colors generated by the projectorsubsystem 310 are not spatially separated from each other, and the ICGs342, 352, and 362 in the blue, green, and red waveguides 340, 350, and360 are laterally aligned with respect to each other. Thus, image lightpass through each waveguide sequentially in a “serial” fashion. Theeyepiece 330 may further include a first wavelength-selective opticalelement 392 positioned between the ICG 342 in the blue waveguide 340 andthe ICG 352 in the green waveguide 350, and a secondwavelength-selective optical element 394 positioned between the ICG 352in the green waveguide 350 and the ICG 362 in the red waveguide 360. Thefirst and second wavelength-selective optical elements 392 and 394 may,for instance, represent wavelength-selective optical filters (i.e.,optical elements that selectively transmit light in a particular rangeof wavelengths) and/or wavelength-selective optical reflectors (i.e.,mirrors and other optical elements that selectively reflect light in aparticular range of wavelengths). As described in further detail below,a dichroic filter is one example of an optical element configured toboth selectively transmit and reflect light on the basis of wavelength.In the following, the first and second wavelength-selective opticalelements 392 and 394 may also be referred to as “optical filter 392” and“optical filter 394,” respectively. Similarly, otherwavelength-selective optical elements described with reference to any ofFIGS. 4-14 may also be referred to herein as “optical filters.”

As illustrated in FIG. 3, image light in all three colors is incident onthe ICG 342 in the blue waveguide 340. The ICG 342 in the blue waveguide340 may couple a portion of the image light primarily in the bluewavelength range into the blue waveguide 340 to be guided toward the OPEregion 344. The ICG 342 in the blue waveguide 340 may also couple asmall amount of green image light, and even a smaller amount of redlight, into the blue waveguide 340, as will be discussed further later.Image light that is not coupled into the blue waveguide 340 istransmitted through the blue waveguide 340 and incident on the firstoptical filter 392. The first optical filter 392 may be configured tohave a high transmittance value in the green and red wavelength ranges,and a low transmittance value in the blue wavelength range. Therefore,image light transmitted by the first optical filter 392 and incident onthe ICG 352 in the green waveguide 350 may contain primarily green imagelight and red image light, and very little or no blue image light.

Still referring to FIG. 3, the ICG 352 in the green waveguide 350 maycouple a portion of the image light primarily in the green wavelengthrange into the green waveguide 350 to be guided toward the OPE region354. The ICG 352 in the green waveguide 350 may also couple a smallamount of red image light into the green waveguide 350, as will bediscussed further later. Image light that is not coupled into the greenwaveguide 350 may be transmitted through the green waveguide 350 andincident on the second optical filter 394. The second optical filter 394may be configured to have a high transmittance value in the redwavelength range, and a low transmittance value in the green and bluewavelength ranges. Therefore, image light transmitted by the secondoptical filter 394 and incident on the ICG 362 in the red waveguide 360may contain primarily red image light, and very little or no green imagelight and blue image light. The ICG 362 in the red waveguide 360 maycouple a portion of the image light primarily in the red wavelengthrange into the red waveguide 360 to be guided toward the OPE region 364.

FIG. 4 illustrates schematically a plan view of an eyepiece 400according to an embodiment of the present invention. The eyepiece 400may include a blue waveguide 440, a green waveguide 450, and a redwaveguide 460 stacked in adjacent lateral planes. Each waveguide 440,450, or 460 may include an ICG region 410, an OPE region 420, and a EPEregion 430. The ICG regions 410 for the three waveguides 440, 450, and460 may be disposed in the same lateral position, and are thus stackedalong the same optical path. A first optical filter 492 may bepositioned between the ICG 410 of the blue waveguide 440 and the ICG 410of the green waveguide 450. A second optical filter 492 may bepositioned between the ICG 410 of the green waveguide 450 and the ICG410 of the red waveguide 460. The eyepiece 400 illustrated in FIG. 4 mayfunction substantially as described above with respect to FIG. 3. Theeyepiece illustrated in FIGS. 3 and 4 may have a smaller form factor ascompared to the eyepiece 230 illustrated FIG. 2, because the ICGs 410 inthe three waveguides 440, 450, and 460 are disposed at the same lateralposition instead of laterally displaced from each other.

FIG. 5 illustrates schematically a partial cross-sectional view of aneyepiece 500 according to an embodiment of the present invention. Theeyepiece 500 may include a first planar waveguide 510, disposed in afirst lateral plane. The first waveguide 510 may include a first lateralregion (labeled as X10) and a second lateral region (labeled as X11).The first lateral region (X10) may be disposed at a lateral position andconfigured to receive image light (X02) incident on a first lateralsurface thereof. The image light (X02) may include image light in afirst wavelength range, image light in a second wavelength range, andimage light in the third wavelength range. For example, the firstwavelength range may be centered at about 462 nm wavelengthcorresponding to blue light, the second wavelength range may be centeredat about 528 nm wavelength corresponding to green light, and the thirdwavelength range may be centered at about 635 nm wavelengthcorresponding to red light.

The eyepiece 500 may further include a first diffractive optical element(DOE) 512 optically coupled to the first lateral region (X10) of thefirst waveguide 510. The first DOE 512 may include an incoupling grating(ICG) formed either on the first surface of the first waveguide 510 (asshown in FIG. 5) or a second surface of the first waveguide 510 oppositeto the first surface. The first DOE may be configured to diffract imagelight in the first wavelength range, e.g., blue image light (X14), intothe first waveguide 510 to be guided toward the second lateral region(X11) of the first waveguide 510. The second lateral region (X11) may bea region between the ICG and an OPE (not shown). A portion of the imagelight (X12) that is not coupled into the first waveguide 510 may betransmitted through the first waveguide 510.

The eyepiece 500 may further include a first optical filter 520positioned in a second lateral plane adjacent the first lateral plane atthe same lateral position as the first lateral region (X10) of the firstwaveguide 510. The first optical filter 520 may be configured to receivethe portion of the image light (X12) transmitted through the firstwaveguide 510. In one embodiment, the first optical filter 520 may beconfigured as a long-pass filter such that it has high transmittancevalues for the wavelength ranges corresponding to green and red light,and low transmittance values for the wavelength range corresponding toblue light. Thus, image light transmitted by the first optical filter520 (X22) may contain primarily green and red image light.

The eyepiece 500 may further include a second planar waveguide 530positioned in a third lateral plane adjacent the second lateral plane.The second waveguide 530 may have a first lateral region (X30) and asecond lateral region (X31). The first lateral region (X30) may bedisposed at the same lateral position as the first lateral region of thefirst waveguide 510, and may be configured to receive image lighttransmitted by the first optical filter 520 (X22) incident on a firstlateral surface thereof.

The eyepiece may further include a second diffractive optical element(DOE) 532 optically coupled to the first lateral region (X30) of thesecond waveguide 530. The second DOE 532 may include an incouplinggrating (ICG) formed either on the first surface of the second waveguide530 (as shown in FIG. 5) or a second surface of the second waveguide 530opposite to the first surface. The second DOE 532 may be configured todiffract image light in the second wavelength range, e.g., green imagelight (X34), into the second waveguide 530 to be guided toward thesecond lateral region (X31) of the second waveguide 530. The secondlateral region (X31) may be a region between the ICG and an OPE (notshown). A portion of the image light (X32) that is not coupled into thesecond waveguide 530 may be transmitted through the second waveguide530.

The eyepiece may further include a second optical filter 540 positionedin a fourth lateral plane adjacent the third lateral plane at the samelateral position as the first lateral region (X30) of the secondwaveguide 530. The second optical filter 540 may be configured toreceive the portion of the image light (X32) transmitted through thesecond waveguide 530. In one embodiment, the second optical filter 540may be configured as a long-pass filter such that it has hightransmittance values for the wavelength range corresponding to redlight, and low transmittance values for the wavelength rangescorresponding to blue and green light. Thus, image light transmitted bythe second optical filter 540 (X42) may contain primarily red imagelight.

The eyepiece 500 may further include a third planar waveguide 550positioned in a fifth lateral plane adjacent the fourth lateral plane.The third waveguide 550 may have a first lateral region (X50) and asecond lateral region (X51). The first lateral region (X50) may bedisposed at the same lateral position as the first lateral region (X30)of the second waveguide 530, and may be configured to receive imagelight transmitted by the second optical filter 540 (X42) incident on afirst lateral surface thereof.

The eyepiece 500 may further include a third diffractive optical element(DOE) 552 optically coupled to the first lateral region (X50) of thethird waveguide 550. The third DOE 552 may include an incoupling grating(ICG) (not shown) formed either on the first surface of the thirdwaveguide 550 (as shown in FIG. 5) or a second surface of the thirdwaveguide 550 opposite to the first surface. The third DOE 552 may beconfigured to diffract image light in the third wavelength range, e.g.,red image light (X54), into the third waveguide 550 to be guided towardthe second lateral region (X51) of the third waveguide 550. The secondlateral region (X51) may be a region between the ICG and an OPE (notshown). A portion of the image light (X52) that is not coupled into thethird waveguide 550 may be transmitted through the third waveguide 550.

According to some other embodiments, the order of the red-green-bluewaveguides 510, 530, and 550 may be different from that illustrated inFIG. 5. Further, the eyepiece 500 may include fewer than threewaveguides (e.g., two waveguides), or more than three waveguides (e.g.,nine waveguides, three for each color), according to some embodiments.In some embodiments, the eyepiece 500 may include waveguides for colorsother than red, green, and blue. For example, it may include waveguidesfor magenta and cyan, in place of or in addition to red, green, andblue.

In some embodiments, the first optical filter 520 may be configured as adichroic long-pass filter that transmits green and red light, andreflects blue light. Thus, a portion of the image light (X12)transmitted through the first waveguide 510 that is in the bluewavelength range (X24) may be reflected back toward the first waveguide510 and be diffracted by the first DOE into the first waveguide 510 tobe guided to the OPE and EPE in the first waveguide 510, and be outputto the viewer. As such, the brightness and contrast of the blue lightfield that is output to the viewer may be enhanced.

Similarly, the second optical filter 540 may be configured as a dichroiclong-pass filter that transmits red light, and reflects blue and greenlight. Thus, a portion of the image light (X32) transmitted through thesecond waveguide 530 that is in the green wavelength range (X44) may bereflected back toward the second waveguide 530 and be diffracted by thesecond DOE into the second waveguide 530 to be guided to the OPE and EPEin the second waveguide 530, and be output to the viewer. As such, thebrightness and contrast of the green light field that is output to theviewer may be enhanced.

In some embodiments, the eyepiece may further include an opticalreflector 560 positioned in a sixth lateral plane adjacent the fifthlateral plane at the same lateral position as the second lateral region(X50) of the third waveguide 550. Much like the abovementioned dichroiclong-pass filters, the optical reflector 560 may be configured toreflect image light transmitted through the third waveguide 550 (X52)back toward the third waveguide 550. A portion of the image lightreflected by the optical reflector 560 (X64) in the red wavelength rangemay be diffracted by the third DOE into the third waveguide 550 to beguided to the OPE and EPE of the third waveguide 550, and be output tothe viewer. In some examples, the optical reflector 560 may beimplemented as a wavelength-selective optical element, such as adichroic filter configured to reflect light in at least the redwavelength range. In other examples, the optical reflector 560 may beimplemented as a mirror or other optical element configured to reflect arelatively wide range of wavelengths. In either case, the brightness andcontrast of the red light field that is output to the viewer may beenhanced.

FIGS. 6A-6D illustrate some example images formed by an eyepiece withoutdichroic filters according to an embodiment. FIG. 6A is an image formedby image light that includes red image light, green image light, andblue image light. FIGS. 6B-6D are images formed by red image light,green image light, and blue image light, respectively. FIG. 6E-6Hillustrate some example images formed by an eyepiece with dichroicfilters, such as the eyepiece 500 illustrated in FIG. 5, according to anembodiment. FIG. 6E is an image formed by image light that includes redimage light, green image light, and blue image light. FIGS. 6F-6H areimages formed by red image light, green image light, and blue imagelight, respectively. As can be seen, the images formed by an eyepiecewith dichroic filters may be brighter than those formed by an eyepiecewithout dichroic filters. Indeed, the reflective properties of dichroicfilters can serve to enhance brightness in waveguide-based eyepieces.

FIG. 7 illustrates schematically a transmittance/reflectance curve forthe first optical filter 520 according to an embodiment of the presentinvention. The first optical filter 520 may be configured as a long-passfilter that has high transmittance values (e.g., close to 100%) and lowreflectance values (e.g., close to 0%) for wavelengths longer than athreshold wavelength (e.g., 510 nm), and low transmittance values (e.g.,close to 0%) and high reflectance values (e.g., close to 100%) forwavelengths shorter than the threshold wavelength.

In some embodiments, the first optical filter 520 may be configured tohave transmittance values greater than about 90% for wavelengths longerthan a threshold wavelength (e.g., 510 nm), and transmittance valuesless than about 10% for wavelengths shorter than the thresholdwavelength. In some other embodiments, the first optical filter 520 maybe configured to have transmittance values greater than about 80% forwavelengths longer than a threshold wavelength (e.g., 510 nm), andtransmittance values less than about 20% for wavelengths shorter thanthe threshold wavelength. The first optical filter 520 may have othertransmittance value ranges. Color contrast may vary depending on thetransmittance value ranges.

The first optical filter 520 may include, for example, a multi-layerthin-film filter. The transmittance/reflectance curve of a multi-layerthin-film filter is typically sensitive to angle of incidence. Forexample, the first optical filter 520 may be designed to have thetransmittance/reflectance curve represented by the solid line 710 for azero-degree angle of incidence (i.e., normal incidence), where thethreshold wavelength is about 510 nm. For increasing angle of incidence,the threshold wavelength may shift to shorter wavelengths. For example,the threshold wavelength may shift to about 459 nm for a 45-degree angleof incidence as indicated by the dashed line 720. In some embodiments,the first optical filter 520 may be designed such that the thresholdwavelength stays below the center wavelength of green image light (e.g.,528 nm) and above the center wavelength of blue image light (e.g., 462nm) for a predetermined range of angles of incidence. In one embodiment,the predetermined range of angles of incidence may be from about zerodegree to about 45 degrees, for a 90-degree field of view (FOV). Inanother embodiment, the predetermined range of angles of incidence maybe from about zero degree to about 25 degrees, for a 50-degree FOV. Suchfilter design may enable angle-insensitive operation for the firstoptical filter 520. That is, the first optical filter 520 will transmitgreen and red light and reflect blue light, as long as the angle ofincidence of the image light is within the predetermined range.

FIG. 8 illustrates schematically a transmittance/reflectance curve forthe first optical filter 520 according to another embodiment of thepresent invention. Here, the first optical filter 520 may be designed tohave the transmittance/reflectance curve represented by the solid line810 for a 45-degree angle of incidence, where the threshold wavelengthis about 459 nm. For decreasing angle of incidence, the thresholdwavelength may shift to longer wavelengths. For example, the thresholdwavelength may shift to about 510 nm for a zero-degree angle ofincidence as indicated by the dashed line 820. The first optical filter520 may be designed such that the threshold wavelength stays below thecenter wavelength of green image light (e.g., 528 nm) and above thecenter wavelength of blue image light (e.g., 462 nm) for a predeterminedrange of angles of incidence, for angle-insensitive operation.

The second optical filter 540 may also be designed for angle-insensitiveoperation. For example, the second optical filter 540 may be designed asa long-pass filter that has a threshold wavelength below the centerwavelength of red image light (e.g., 635 nm) and above the centerwavelength of green image light (e.g., 528 nm) for a predetermined rangeof angles of incidence.

Referring to FIG. 5, in some other embodiments, the red-green-bluewaveguides 510, 530, and 550 may be ordered differently. For example,the first waveguide 510 may be configured as a red waveguide, the secondwaveguide 530 may be configured as a green waveguide, and the thirdwaveguide 550 may be configured as a blue waveguide. In that case, thefirst optical filter 520 may be configured as a short-pass filter thathas high transmittance values in the blue and green wavelength rangesand a low transmittance value in the red wavelength range. Similarly,the second optical filter 550 may be configured as a short-pass filterthat has a high transmittance value in the blue wavelength range and lowtransmittance values in the green and red wavelength ranges.

As another example, the first waveguide 510 may be configured as a bluewaveguide, the second waveguide 530 may be configured as a redwaveguide, and the third waveguide 550 may be configured as a greenwaveguide. In that case, the first optical filter 520 may be configuredas a long-pass filter that has high transmittance values in the greenand red wavelength ranges and a low transmittance value in the bluewavelength range. The second optical filter 540 may be configured as ashort-pass filter that has a high transmittance value in the greenwavelength range and a low transmittance value in the red wavelengthrange.

Referring to FIG. 5, as described above, the first DOE 512 (alsoreferred to as an ICG) coupled to the first lateral region (X10) of thefirst waveguide 510 may be designed to diffract primarily blue lightinto the first waveguide 510. In practice, the first DOE 512 may alsodiffract (i.e., cross-couple) a small amount of green light into thefirst waveguide 510. FIG. 9A illustrates this situation. There, blueimage light, green image light, and red image light are incident on thefirst waveguide 510. While the first DOE 512 diffracts primarily bluelight into the first waveguide 510 to be guided toward the secondlateral region (X11), a small amount of green image light may also bediffracted by the first DOE 512 into the first waveguide 510.

Similarly, the second DOE 532 coupled to the first lateral region (X30)of the second waveguide 530 may be designed to diffract primarily greenlight into the second waveguide 530. In practice, the second DOE 532 mayalso cross-couple a small amount of red light into the second waveguide530. FIG. 10 illustrates this situation. There, green image light andred image light may be transmitted by the long-pass filter 520 andincident on the second waveguide 530. While the second DOE 532 diffractsprimarily green light into the second waveguide 530 to be guided towardthe second lateral region (X31), a small amount of red image light mayalso be diffracted by the second DOE into the second waveguide 530.

FIGS. 11A-11D illustrate the wavelength “cross-coupling” effect. FIG.11A shows an image of a blue light field formed by a blue waveguide.FIG. 11B shows an image of a green light field that is cross-coupled bythe blue waveguide. FIG. 11C shows an image of a green light fieldformed by a green waveguide. FIG. 11D shows an image of a red lightfield that is cross-coupled by the green waveguide.

According to an embodiment of the present invention, the first waveguide510 may include a first short-pass filter 518 coupled to the secondlateral region (X11) of the first waveguide 510, as illustrated in FIG.9A. The first short-pass filter 518 may be configured to pass blue lightand absorb green light, so that the green image light cross-coupled intothe first waveguide 510 may be absorbed by the first short-pass filter518 and thus may be prevented from propagating to the OPE and the EPEregions of the first waveguide 510.

According to an embodiment, the second waveguide 530 may also include asecond short-pass filter 538 coupled to the second lateral region (X31)of the second waveguide 530, as illustrated in FIG. 10. The secondshort-pass filter 538 may be configured to pass green light and absorbred light, so that the red image light cross-coupled into the secondwaveguide 530 may be absorbed by the second short-pass filter 538 andthus may be prevented from propagating to the OPE and the EPE regions ofthe second waveguide 530.

FIG. 9B illustrates schematically a cross-sectional view of theshort-pass filter 518 according to an embodiment of the presentinvention. The short-pass filter 518 may be disposed on an outer surfaceof the second lateral region (X11) of the first waveguide 510. In someembodiments, the short-pass filter 518 may include a dichroic layer 910disposed on the outer surface of the first waveguide 510. The dichroiclayer 910 may include, for example, a multi-layer thin-film designed tohave a transmittance/reflectance curve similar to that illustrated inFIG. 7, where the threshold wavelength is below the center wavelength ofgreen image light (e.g., 528 nm) and above the center wavelength of blueimage light (e.g., 462 nm) for a predetermined range of angle ofincidence. As such, the dichroic layer 910 may reflect blue image lightincident thereon back into the first waveguide 510 to be guided towardthe EPE region, and transmit green image light incident thereon. Theshort-pass filter 518 may further include a terminating substrate 920(e.g., a glass layer) disposed on the dichroic layer 910, and anabsorptive layer 930 disposed on the terminating substrate 920. Theabsorptive layer 930 may be configured to absorb light transmitted bythe dichroic layer 910.

FIG. 12A illustrates schematically a waveguide 1200 according to someembodiments. The waveguide 1200 may include a first lateral region 1202and a second lateral region 1204. The waveguide 1200 may also include adiffractive optical element (DOE) 1203 optically coupled to the firstlateral region 1202, and configured to diffract a portion of theincident light Y02 a into the waveguide 1200. For example, the DOE 1203may be designed to diffract primarily blue light into the firstwaveguide 1200. In practice, the DOE may also diffract (i.e.,cross-couple) a small amount of green light into the first waveguide1200, as discussed above.

The waveguide 1200 may also include a short-pass filter 1210 coupled tothe second lateral region 1204 of the waveguide 1200. The short-passfilter 1210 may include particles with index-matched characteristicsembedded into the waveguide 1200, for example by a substrate dopingprocess. The particles may absorb, for example, green light or lighthaving a wavelength longer than that of blue light, and transmit bluelight. In some embodiments, index matching may not be a strictrequirement. In such cases, light may refract at the interfaces betweenthe particles and the waveguide medium, but may nevertheless continue topropagate at the original angles. It may be desirable to minimize thescattering at points of discontinuity.

FIG. 12B illustrates schematically a waveguide 1200 according some otherembodiments. The waveguide 1200 may include a short-pass filter 1220coupled to the second lateral region 1204 of the waveguide 1200. Here,the short-pass filter 1220 may include a cavity inside the secondlateral region 1204 of the waveguide 1200, where a top surface of thecavity is flush with the outer surface of the waveguide 1200. The cavitymay be filled with an index-matched dye that absorbs green light orlight having a wavelength longer than that of blue light. In someembodiments, instead of making and filling a cavity, the filter can becreated by diffusing a dye through the surface of the waveguide 1200,producing a partially or fully dyed (or “doped”) volume within thewaveguide 1200. In one embodiment, the refractive index of the dye maybe matched to the refractive index of the waveguide 1200, so that thedye does not affect the propagation of blue image light in the waveguide1200 by total internal reflection (TIR). In some other embodiments, someindex mismatch may be allowed so long as it does not affect thepropagation, and scattering at the interfaces is controlled to somedegree.

FIG. 12C illustrates schematically a waveguide 1200 according somefurther embodiments. The waveguide 1200 may include a layer of dye 1230applied to the outer surface of the second lateral region 1204 of thewaveguide 1200. The layer of dye (Y13 c) may absorb green light or lighthaving a wavelength longer than that of blue light, and reflects bluelight.

FIG. 13A illustrates a partial cross-sectional view of an eyepieceaccording to another embodiment of the present invention. The eyepiecemay include a first planar waveguide 1310 positioned in a first lateralplane, a second planar waveguide 1340 positioned in a second lateralplane adjacent the first lateral plane, and a third planar waveguide1370 positioned in a third lateral plane adjacent the third lateralplane. Input image light is split into two optical paths, where blue andred image light is incident on the eyepiece at a first lateral position,and green image light is incident on the eyepiece at a second lateralposition displaced from the first lateral position.

The eyepiece may further include a first diffractive optical element(DOE) 1320, such as an incoupling grating (ICG), disposed on a firstsurface of the first waveguide 1310 at the second lateral position. Thefirst DOE is configured to receive and diffract a portion of the greenimage light incident thereon into the first waveguide 1310 to be guidedto the OPE and the EPE region of the first waveguide 1310. The eyepiecemay further include a first optical reflector 1330 disposed on a secondsurface of the first waveguide 1310 at the second lateral position. Insome examples, the optical reflector 1330 may be implemented as awavelength-selective optical element, such as a dichroic filterconfigured to reflect light in at least the green wavelength range. Inother examples, the optical reflector 1330 may be implemented as amirror or other optical element configured to reflect a relatively widerange of wavelengths (e.g., aluminized material). It follows that, ineither case, the first optical reflector 1330 may be configured toreflect green image light that is not coupled into the first waveguide1310 by the first DOE 1320 on the first pass back toward the first DOE1320. A portion of the green image light reflected by the first opticalreflector 1330 may be diffracted by the first DOE 1320 into the firstwaveguide 1310. Therefore, the brightness and contrast of the greenlight field that is output to the viewer may be enhanced.

The eyepiece may further include a second DOE 1350 disposed on the firstsurface of the second waveguide 1340 at the first lateral position. Thesecond DOE 1350 may be configured to receive and diffract a portion ofthe blue image light incident thereon into the second waveguide 1340 tobe guided toward the OPE and the EPE region of the second waveguide1340. The eyepiece may further include an optical filter 1360 (i.e., awavelength-selective optical element) disposed on a second surface ofthe second waveguide 1340 at the first lateral position. The opticalfilter 1360 may include a dichroic long-pass filter configured to have ahigh transmittance value for red image light, and a low transmittancevalue and a high reflectance value for blue image light. Thus, theportion of blue image light that is not coupled into the secondwaveguide 1340 by the second DOE 1350 on the first pass may be reflectedback toward the second DOE 1350 and be coupled into the second waveguide1340 by the second DOE 1350. Therefore, the brightness and contrast ofthe blue light field that is output to the viewer may be enhanced. Redimage light transmitted by the optical filter 1360 is incident on thethird waveguide 1370.

The eyepiece may further include a third DOE 1380 disposed on the firstsurface of the third waveguide 1370 at the first lateral position. Thethird DOE 1380 may be configured to receive and diffract a portion ofthe red image light incident thereon into the third waveguide 1370 to beguided toward the OPE and the EPE region of the third waveguide 1370.The eyepiece may further include a second optical reflector 1390disposed on a second surface of the third waveguide 1370 at the firstlateral position. In some examples, the optical reflector 1390 may beimplemented as a wavelength-selective optical element, such as adichroic filter configured to reflect light in at least the redwavelength range. In other examples, the optical reflector 1390 may beimplemented as a mirror or other optical element configured to reflect arelatively wide range of wavelengths (e.g., aluminized material). Ineither case, the second optical reflector 1390 may be configured toreflect red image light that is not coupled into the third waveguide1370 by the third DOE 1380 on the first pass back toward the third DOE1380. A portion of the red image light reflected by the second opticalreflector 1390 may be diffracted by the third DOE 1380 into the thirdwaveguide 1370. Therefore, the brightness and contrast of the red lightfield that is output to the viewer may be enhanced.

FIG. 13B illustrates a partial cross-sectional view of an eyepieceaccording to a further embodiment of the present invention. The eyepieceillustrated in FIG. 13B is similar to the eyepiece illustrated in FIG.13A, except that the first DOE is disposed on the second surface of thefirst waveguide 1310, the same surface as the first optical reflector1330, the second DOE 1350 is disposed on the second surface of thesecond waveguide 1340, the same surface as the optical filter 1360, andthe third DOE 1380 is disposed on the second surface of the thirdwaveguide 1370, the same surface as the second optical reflector 1390.

FIG. 13C illustrates a partial cross-sectional view of an eyepieceaccording to a yet another embodiment of the present invention. Theeyepiece illustrated in FIG. 13C is similar to the eyepiece illustratedin FIG. 13B, except that the optical filter 1360 is disposed on thefirst surface of the third waveguide 1370.

Each of the embodiments illustrated in FIGS. 13A-13C may have its ownpros and cons. In the embodiment illustrated in FIG. 13A, because thefirst DOE, the second DOE, and the third DOE are formed on the firstsurface of the waveguide, they operate in transmission mode. Incomparison, in the embodiments illustrated in FIGS. 13B and 13C, thefirst DOE, the second DOE, and the third DOE are formed on the secondsurface of the waveguide, and thus operate in reflection mode. The DOEsmay be more efficient in reflection mode than in transmission mode.Having the DOEs aluminized in reflection mode may further increasediffraction efficiency. Having the DOE and the dichroic filter onopposite surfaces may be more challenging to manufacture, as it requirespatterning on both surfaces.

FIG. 14 illustrates schematically a transmittance/reflectance curve ofthe optical filter 1360 according to an embodiment of the presentinvention. The transmittance/reflectance curve of the optical filter1360 is similar to those illustrated in FIGS. 7 and 8, in that itexhibits high transmittance values (e.g., close to 100%) and lowreflectance values (e.g., close to 0%) for wavelengths longer than athreshold value, and low transmittance values (e.g., close to 0%) andhigh reflectance values (e.g., close to 100%) for wavelengths shorterthan the threshold value.

The optical filter 1360 may include a multi-layer thin-film whosetransmittance/reflectance characteristics may be sensitive to angle ofincidence as discussed above. For example, the optical filter 1360 maybe designed to have the transmittance/reflectance curve represented bythe solid line 1410 for an angle of incidence of 45 degrees. Fordecreasing angle of incidence, the rising edge may shift to longerwavelengths. For example, the transmittance/reflectance curve for azero-degree angle of incidence may be represented by the dashed line1420.

As discussed above, to enable angle-insensitive operation for theoptical filter 1360, it may be desirable that the rising edge of thetransmittance/reflectance curve stay below the center wavelength of redimage light (e.g., 635 nm) and above the center wavelength of blue imagelight (e.g., 462 nm) for a predetermined range of angle of incidence(e.g., from about zero degree to about 45 degrees). Here, because onlyblue and red image light is incident on the optical filter 1360, andbecause the center wavelengths of blue image light and red image lightare relatively far apart from each other, the requirement on thetransmittance/reflectance profile can be more relaxed. For example, therising edge of the transmittance/reflectance curve may shift by a largerwavelength range between a zero-degree angle of incidence and a45-degree angle of incidence, as compared to that illustrated in FIGS. 7and 8. Also, the rising edge of the transmittance/reflectance curve maynot need to be as steep as those illustrated in FIGS. 7 and 8. Thus, theeyepiece illustrated in FIGS. 13A-13C may afford a smaller form factoras compared to the eyepiece illustrated in FIG. 2 where theblue-green-red image light is separated into three separate light paths,while having a less stringent requirement for the filter'stransmittance/reflectance profile.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims.

What is claimed is:
 1. An eyepiece for projecting image light to an eye of a viewer, the image light including image light in a first wavelength range centered at a first wavelength and image light in a second wavelength range centered at a second wavelength different from the first wavelength, the eyepiece comprising: a first planar waveguide including a first diffractive optical element (DOE) optically coupled thereto, wherein the first DOE is positioned along an optical path of the image light and configured to couple a portion of the image light in the first wavelength range into the first planar waveguide to be propagated in the first planar waveguide; a first optical filter positioned along the optical path downstream from the first DOE, wherein the first optical filter is configured to attenuate the image light in the first wavelength range incident thereon; a second planar waveguide including a second DOE optically coupled thereto, wherein the second DOE is positioned along the optical path downstream from the first optical filter and configured to couple a portion of the image light in the second wavelength range transmitted through the first optical filter into the second planar waveguide to be propagated in the second planar waveguide; and a second optical filter coupled to the first planar waveguide, wherein the second optical filter is configured to absorb image light in the second wavelength range propagating in the first planar waveguide.
 2. The eyepiece of claim 1 wherein the second wavelength is longer than the first wavelength, and the first optical filter comprises a long-pass filter configured to have a first transmittance value for the first wavelength range that is less than about 10% and a second transmittance value for the second wavelength range that is greater than about 90%.
 3. The eyepiece of claim 2 wherein the second optical filter comprises a short-pass filter configured to have a first transmittance value for the second wavelength range that is less than about 10%, and a second transmittance value for the first wavelength range that is greater than about 90%.
 4. The eyepiece of claim 2 wherein the first optical filter is configured to have the first transmittance value and the second transmittance value for angles of incidence ranging from about zero degrees to about 45 degrees.
 5. The eyepiece of claim 2 wherein the first optical filter comprises a dichroic filter configured to reflect a portion of the image light in the first wavelength range toward the first planar waveguide.
 6. The eyepiece of claim 1 wherein the second wavelength is shorter than the first wavelength, and the first optical filter comprises a short-pass filter configured to have a first transmittance value for the first wavelength range that is less than about 10% and a second transmittance value for the second wavelength range that is greater than about 90%.
 7. The eyepiece of claim 6 wherein the second optical filter comprises a long-pass filter configured to have a first transmittance value for the second wavelength range that is less than about 10%, and a second transmittance value for the first wavelength range that is greater than about 90%.
 8. The eyepiece of claim 1 wherein the second optical filter comprises: a dichroic layer disposed on an outer surface of the first planar waveguide; a terminating substrate disposed on the dichroic layer; and an absorptive layer disposed on the terminating substrate.
 9. The eyepiece of claim 1 wherein each of the first DOE and the second DOE comprises an incoupling grating.
 10. The eyepiece of claim 1 further comprising an optical reflector positioned along the optical path downstream from the second planar waveguide.
 11. A method for projecting image light to an eye of a viewer, the image light including image light in a first wavelength range centered at a first wavelength and image light in a second wavelength range centered at a second wavelength different from the first wavelength, the method comprising: coupling, through a first diffractive optical element (DOE), a portion of the image light in the first wavelength range into a first planar waveguide, wherein the first DOE is positioned along an optical path of the image light in the first wavelength range and optically coupled to the first planar waveguide; attenuating the image light in the first wavelength range through a first optical filter, wherein the first optical filter is positioned along the optical path downstream from the first DOE; transmitting a portion of the image light in the second wavelength range through the first optical filter; coupling, through a second DOE, the transmitted portion of the image light in the second wavelength range into a second planar waveguide, wherein the second DOE is positioned along the optical path of the image light in the second wavelength range downstream from the first optical filter and is optically coupled to the second planar waveguide; and absorbing at least a portion of the image light in the second wavelength range propagating in the first planar waveguide by a second optical filter coupled to the first planar waveguide.
 12. The method of claim 11 wherein the second wavelength is longer than the first wavelength, and the first optical filter comprises a long-pass filter configured to have a first transmittance value for the first wavelength range that is less than about 10% and a second transmittance value for the second wavelength range that is greater than about 90%.
 13. The method of claim 12 wherein the second optical filter comprises a short-pass filter configured to have a first transmittance value for the second wavelength range that is less than about 10%, and a second transmittance value for the first wavelength range that is greater than about 90%.
 14. The method of claim 12 wherein the first optical filter is configured to have the first transmittance value and the second transmittance value for angles of incidence ranging from about zero degrees to about 45 degrees.
 15. The method of claim 12 wherein the first optical filter comprises a dichroic filter configured to reflect a portion of the image light in the first wavelength range toward the first planar waveguide.
 16. The method of claim 11 wherein the second wavelength is shorter than the first wavelength, and the first optical filter comprises a short-pass filter configured to have a first transmittance value for the first wavelength range that is less than about 10% and a second transmittance value for the second wavelength range that is greater than about 90%.
 17. The method of claim 16 wherein the second optical filter comprises a long-pass filter configured to have a first transmittance value for the second wavelength range that is less than about 10%, and a second transmittance value for the first wavelength range that is greater than about 90%.
 18. The method of claim 11 wherein the second optical filter comprises: a dichroic layer disposed on an outer surface of the first planar waveguide; a terminating substrate disposed on the dichroic layer; and an absorptive layer disposed on the terminating substrate.
 19. The method of claim 11 wherein each of the first DOE and the second DOE comprises an incoupling grating.
 20. The method of claim 11 further comprising reflecting image light in the second wavelength range transmitted through the second planar waveguide by an optical reflector positioned along the optical path downstream from the second planar waveguide. 